The present invention is a process for determining the organoamines concentrations in chlorosilane. The process comprises separating a mixture comprising as a major portion a chlorosilane and as a minor portion an organoamine having a boiling point less than or about the boiling point of the chlorosilane, where the organoamine is complexed in situ in the chlorosilane forming an organoamine complex which vaporizes at a temperature greater than the boiling point of the chlorosilane, into a vapor phase comprising the chlorosilane and a solid phase comprising the organoamine complex. An aqueous solution of the solid phase is formed and analyzed by ion exchange chromatography for organoamine content. The present invention is particularly useful for analyzing chlorosilanes for dimethylamine and trimethylamine where the chlorosilane or residual hydrogen chloride in the chlorosilane serves to complex the organoamine.
The production of semiconductor grade silicon typically involves the decomposition of a chlorosilane gas such as trichlorosilane to deposit silicon onto a heated element by a process referred to as chemical vapor deposition (CVD). Continuous improvement in the technology to concentrate integrated circuits on silicon wafers has created a continuing need to improve the purity of chlorosilane gas used in the deposition process. Organoamines represent a potential source of detrimental contaminates in chlorosilanes. Therefore, it is desirable to have a reliable method for measuring the levels of organoamines in chlorosilanes.
Typical commercial processes for manufacturing chlorosilanes involve the contact of hydrogen chloride with silicon metalloid in a fluidized-bed reactor. The product of this reaction is a mixture comprising chlorosilanes, including silane (SiH.sub.4), chlorosilane, dichlorosilane, trichlorosilane, and tetrachlorosilane. In addition the mixture can contain hydrogen chloride. In certain incidences it can be desirable to disproportionate this mixture to increase the proportion of commercially more desirable chlorosilanes, such as trichlorosilane. In addition during conduct of the CVD process for making semi-conductor grade silicon, a gaseous product is produced comprising more reduced chlorosilanes, that is chlorosilanes having more hydrogen. It is often desirable to disproportionate more reduced chlorosilanes to more commercially desirable chlorosilanes such as trichlorosilane.
The disproportionation of chlorosilanes occurs slowly in the absence of a catalyst. However, in the presence of a catalyst such as an organoamine the disproportionation can occur readily. Preferred is when the organoamine is attached to a solid support. Examples of the use of supported organoamine catalyst in the disproportionation of chlorosilanes are described in Litteral, U.S. Pat. No. 4,113,845; Bakay, U.S. Pat. No. 3,968,199; Bakay, U.S. Pat. No. 3,928,542; Lepage et al., U.S. Pat. No. 4,548,917; and Seth, U.S. Pat. No. 4,395,389. Also, described in Arvidson, et al., U.S. Pat. No. 5,118,485, is a process where lower-boiling silanes comprising an effluent gas from a CVD process can be disproportionated in the presence of a nitrogen containing material on a solid support. The present invention is especially useful for analyzing the product of such disproportionation processes for organoamines resulting from the decomposition of the nitrogen containing catalysts.
The analysis of aqueous solutions by ion exchange chromatography (IEC) for organoamines has been described. For example, Bouyoucos, Analytical Chemistry 49:401-403, 1977, describes and IEC technique for analyzing an aqueous solution for determination of ammonia, monomethylamine, dimethylamine, and trimethylamine. Buechele et al., Anal. Chem. 54:2114-2115, 1982, describe a process for analyzing an aqueous solution for ethylenediamine by IEC. Bouyoucos et al., Am. Ind. Hyg. Assoc. J. 44:119-122, 1983, describe an IEC process for analyzing an aqueous solution for ammonia, monomethylamine, dimethylamine, and trimethylamine, where the amine was collected from air by adsorption on to silica gel. Gilbert et al., Anal. Chem. 56:106-109, 1984, report a process for IEC determination of morpholine and cyclohexylamine in aqueous solutions containing ammonia and hydrazine. Rich et al., U.S. Pat. No. 4,242,097, describe a method and apparatus for quantitative analysis of weakly ionized anions, such as organoamines, by IEC. Daigle et al., Chromatographia 32:143-147, 1991, describes a process for determining the concentration of various organoamines in an aqueous solution by IEC. Diagle et al. reported detection limits as low as tenths of parts per million (ppm) of the organoamines.
The ability of chlorosilanes to complex with trimethylamine at low temperatures is reported by Burg, J. Am. Chem. Soc. 76:2674-2675, 1954. Burg reports that the complex of tetramethylsilane with trimethylamine begins to dissociate at about -54.1.degree. C., the complex of trichlorosilane with trimethylamine begins to dissociate at about -30.degree. C., and the complex of dichlorosilane with trimethylamine may be stable at temperatures below about 38.degree. C. Ring et al., J. Am. Chem. Soc. 93:265-267, 1971, reports the formation of a 1:1 adduct of trichlorosilane with trimethylamine by multiple condensations procedures at a temperature of -78.degree. C. The salt HN(CH.sub.3).sub.3.sup.+ SiCl.sub.3.sup.- was reported to be thermally stable with evidence of some decomposition at 38.degree. C. Jeng et al., Inorg. Chem. 29:837-842, 1990, report a matrix isolation technique and twin-jet deposition method to isolate and characterize the reaction products of the codeposition of trichlorosilane with bases containing nitrogen. Jeng et al. conclude that the codeposition of the tested bases containing nitrogen with trichlorosilane led to the formation of isolated 1:1 molecular complexes. Jeng et al., concluded that the base interacted with the silicon center on trichlorosilane to form a coordinated complex.
The problem to be solved by the present method is how to analyze a chlorosilane for organoamine contaminants, where at least one of the organoamines has a boiling point less than or about the boiling point of the chlorosilane and the chlorosilane may contain organoamine complexes capable of sublimation and subsequents condensing causing fowling of analytical devices.
The ability to analyze the concentration of organoamines in chlorosilanes is complicated by the reactivity of chlorosilanes. Chlorosilanes hydrolyze on contact with water, therefore techniques such as ion exchange chromatography using an aqueous carrier will not work. In addition when subject to high heat such as in atomic emission or atomic adsorption, the chlorosilane decompose into particulate silica. In general techniques such as infrared adsorption and gas chromatography fail because complexes of the organoamine collect in the equipment making the results unreliable.
Unexpectly, the present inventors have found that organoamines having a boiling point less than or about the boiling point of the chlorosilane to be analyzed can readily complex with a Lewis acid. The resulting organoamine and Lewis acid complex have sufficient stability to allow the chlorosilane to be evaporated leaving a residue comprising the complexed organoamine. An aqueous solution of the residue comprising the complexed organoamine can then be formed and analyzed for organoamine content by ion exchange chromatography.